# One-electron atoms in screened modified gravity

## TL;DR

This paper explores how one-electron atoms behave in scalar-tensor modified gravity models, revealing significant corrections to atomic spectra, new constraints on scalar-photon interactions, and implications for fundamental physics tests.

## Contribution

It provides a detailed analysis of atomic structure modifications, introduces new bounds on scalar couplings, and studies hyperfine effects and equivalence principle violations in these models.

## Key findings

- Large increase in predicted Lamb shift contributions.
- New constraints on scalar-photon coupling independent of matter coupling.
- Inclusion of hyperfine structure and 21 cm line effects.

## Abstract

In a large class of scalar-tensor theories that are potential candidates for dark energy, a nonminimal coupling between the scalar and the photon is possible. The presence of such an interaction grants us the exciting prospect of directly observing dark sector phenomenology in the electromagnetic spectrum. This paper investigates the behavior of one-electron atoms in this class of modified gravity models, exploring their viability as probes of deviations from general relativity in both laboratory and astrophysical settings. Building heavily on earlier studies, our main contribution is threefold: A thorough analysis finds additional fine-structure corrections previously unaccounted for, which now predict a contribution to the Lamb shift that is larger by nearly 4 orders of magnitude. In addition, they also predict a scalar-mediated photon-photon interaction, which now constrains the scalar's coupling to the photon independently of the matter coupling. This was not previously possible with atomic precision tests. Our updated constraints are $\log_{10}\beta_m \lesssim 13.4$ and $\log_{10}\beta_\gamma \lesssim 19.0$ for the matter and photon coupling, respectively, although these remain uncompetitive with bounds from other experiments. Second, we include the effects of the nuclear magnetic moment, allowing for the study of hyperfine structure and the 21 cm line, which hitherto have been unexplored in this context. Finally, we also examine how a background scalar leads to equivalence principle violations.